<form action="<?php bloginfo('siteurl'); ?>" id="searchform" method="get">
  <input type="submit" class="icon magnify" value="" />
  <input type="hidden" value="4,1,5,11,6,7,8,9,10,19" name="cat" id="scat" />
  <input type="text" id="s" name="s" placeholder="Search" value="" />
</form>